Just the first nationwide trial run for the country's civil defenses. Grown wise through their vulnerable position in the heart of Europe, the Swiss are cautiously preparing for the future which seems to have become a bit shakier since talk of limited nuclear wars. Their simple philosophy: If you want to survive, better start seeing how to - now.

From now on, sirens in Switzerland's 3,000 communities will screech their warning twice a year - in February and September.

Switzerland has one of the most highly developed civil defense systems in the world. A network of nuclear-resistant, underground cellars cover the country, along with subterranean hospitals and command posts.

On the penultimate page of Swiss telephone books three different alarms are carefully explained:

* Radioactive nuclear alarm: two minutes long, close doors and windows, head for the nearest civil defense cellar.

* Flood alarm: six minutes long, warning people to run to the nearest safe ground.

* General alarm: one minute long; turn on radio and wait for instructions.

Not everybody agreed with the test alarm. Zurich's Workers Party protested with paper bags filled with a piece of coal and reading ''Reagan paper bag. . .contains a piece of Europe after it was defended by the United States.''

However, most people disagreed with the anti-lobby, believing that a go at survival is well worth the effort.

In Erstfeld, a tiny railway town in the heart of William Tell country, the usual crowd gathered in the ''Hirschen'' pub at midday. At 1.30 p.m., a slow wail echoed down the main street. There was no reaction. Everyone looked complacent.

''News about the alarm has come over the radio for days. We have to be ready if there is a nuclear catastrophe. Of course, you know nothing might happen, but better to be on the alert if it does,'' said Maya Trutman, the innkeeper's wife. While serving a couple of wienerschnitzels, she went on to explain why Erstfeld took the national event in its stride.

''Every Saturday at midday we have an alarm going off to practice in case the two dikes above the village break. Then I known that I have exactly 10 minutes to reach safe ground.

''But now it is a good idea that the whole nation is getting some practice.''